Normally, semiconductor device functions are only performed in a very shallow region on the front surface of silicon wafers, the bulk of the silicon wafer serving only as a mechanical carrier. For more demanding semiconductor applications, such as complex integrated circuits including microprocessors, thin-film silicon-on-insulator (SOI) technology has been established that electrically isolates the thinner active device layer from the bulk of the silicon wafer.
Most methods for making such thin film SOI wafers rely on the relatively slow and expensive processes of ion-implantation. One of the oldest of these implantation methods implants a high dose of oxygen ions into a silicon wafer and then transforms this oxygen into an internal quartz (SiO2) layer by a series of high temperature anneals at temperatures close to the melting point of silicon. Beyond the drawback of relatively high cost, this method also results in reduced crystalline quality as a result of the high dose implantation and the extended high temperature anneals, both of which introduce crystalline defects and contamination into the active device region.
It is well known that the introduction of hydrogen into a metal causes embrittlement and breakage due to the formation of internal hydrogen micro-bubbles. Similarly, if hydrogen is implanted into a silicon wafer, it can be collapsed by specific heat treatments into an internal micro-bubble layer. This methodology has been used to produce thin film wafers as described in U.S. Pat. No. 5,374,564 to Bruel. In this process, hydrogen ions are implanted into a silicon wafer, and, after bonding this implanted wafer to another wafer, the implanted wafer is separated at the internal bubble layer, leaving a thin film of silicon firmly bonded to the second wafer. The implanted wafer may then be polished and cleaned and thus be reused for another cycle. A major drawback of this method is the cost of using ion-beam implanters to deposit the required dose of hydrogen ions into a well-defined depth underneath the surface. Additionally, the wafer has to be kept cool during implantation to avoid premature blistering caused by formation of micro-bubbles already present in the wafer. These drawbacks have limited the usefulness of this thin film production methodology.
Plasma immersion ion implantation has been used in an attempt to reduce the high costs of ion-beam implantation as described in U.S. Pat. No. 6,146,979 to Henley et al. Unfortunately, plasma immersion implantation results in a wide energy distribution of incident hydrogen ions resulting in a broader zone in which the implanted hydrogen comes to rest. This typically necessitates about a ten-fold increase in the minimum implant dose of hydrogen ions which damages the thin wafer layer to be lifted off the surface of that wafer resulting in a final SOI wafer of lower quality than wafers obtained using the process described by Bruel. Additionally, any unwanted elements present in the plasma generator will also become implanted contaminants in the wafer. Because of the lower yield and the reduced quality of the final product caused by these problems, the plasma immersion process was replaced with another process that uses multiple epitaxially-grown layers in conjunction with a low dose hydrogen implant.
A further attempt to reduce the cost of ion implantation is described in U.S. Pat. No. 6,368,938 to Usenko et al. This method combines a low dose ion beam implantation process with a second hydrogen diffusion step wherein the implantation damage created in the first step getters the hydrogen from the second step. The hydrogen for the diffusion step is provided from an electrolytic cell. This second step, however, is difficult to control and can take a long time to reach saturation. For these reasons, this process has not been used for large-scale commercial production of SOI wafers.
All of the present ion-implantation based processes are very expensive due to the use of sophisticated ion beam implanters. The dose requirements and the limitations on beam current require long process times such that the cost of this process step alone can be a multiple of the cost of a regular substrate.
Therefore, there is a need for new methods of producing thin layer semiconductors in which the lift-off of surface films is achieved with lower costs while maintaining or improving the crystalline quality of the product.